This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to species (e.g. water/fat) signal separation in MRI.
Magnetic resonance (MR) imaging is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During a MR experiment, each nuclear spin responds to four different effects—precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MR experiments, a combination of these effects occurs periodically.
Refocused steady-state free precession (SSFP) sequences have recently gained popularity in magnetic resonance imaging, due to improved gradient hardware. SSFP imaging provides high signal and good contrast in short scan times.
As illustrated in FIGS. 1A, 1B, a refocused SSFP sequence includes a single RF excitation which is repeated periodically. All gradients used for slice selection or imaging are fully rewound over each repetitive time, TR. In the steady-state, the magnetization at points a and d is the same.
Magnetization is tipped about a traverse axis through an angle α. Between excitations, the magnetization undergoes a precession by an angle θ=2πΔfTR about the z-axis (direction of B0), where Δf is the tissue off-resonance, and also experiences both T1 and T2 relaxation.
During the sequence each spin is affected by RF pulses, relaxation and free precession. The steady-state magnetization for SSFP is a function of the sequence parameters flip angle (α), repetition time (TR) and echo time (TE) as well as the tissue parameters T1, T2, and resonant frequency shift Δf.
Balanced steady-state free precession (SSFP) imaging sequences offer relatively high signal and rapid 2D or 3D imaging, potentially overcoming two of the prime weaknesses of MR imaging.
However, clinical applications of balanced SSFP are still limited by two main factors. First, the signal is very sensitive to resonant frequency variations that result from static field inhomogeneity, susceptibility variations or chemical shift. Second, balanced SSFP produces a very bright signal from fat which obscures visualization of other structures.
Numerous methods have been presented to suppress the fat signal including methods that generate a fat suppressed steady state, methods that synthesize a fat-suppressed steady state, and methods with transient fat suppression. Alternatively, several methods separate fat and water using multiple acquisitions, or in a single acquisition.
These methods all have advantages and disadvantages when used with balanced SSFP imaging with regard to efficiency, scan time, robustness and accuracy. Many require multiple acquisitions or increased scan time. As a consequence, most methods compromise the SNR efficiency of balanced SSFP. The only exceptions all can fail when fat and water occupy a single voxel. Thus there is still a trade off that must be made when selecting an imaging sequence for a particular application. Furthermore, most of these techniques still need to be compared quantitatively and in a clinical setting.
Dixon SSFP imaging offers high image quality but requires three or more signal acquisitions with varying echo times and a challenging reconstruction.
There are numerous variations of Dixon imaging. Two applications of Dixon fat/water separation with balanced SSFP are IDEAL (iterative decomposition of water and fat using echo asymmetry and least squares estimation) which uses an iterative reconstruction that maintains efficiency by permitting the use of small echo spacings and the in-phase/out-of-phase 2-point approach that is essentially a variation of the linear combination technique. The former of these techniques provides accurate quantitation of fat and water within a voxel. The latter technique requires only 2 acquisitions, but compromises accuracy. A final “Dixon” technique that is of interest here is partially-opposed-phase (POP), which acquires 2 echoes with angular separation 0° and approximately 135°. The present invention here is similar, but with improved efficiency and robustness when applied to balanced SSFP imaging, where a refocused opposed-phase acquisition is possible.